Major efforts are currently being expended to clean up contaminated soils throughout the United States and the world. Such contaminated soils typically arise as industrial products or by-products are spilled either inadvertently or purposely into the environment. Commonly found contaminants are herbicides, pesticides, petroleum products and other hazardous industrial by-products. The time and expense involved in removing these contaminants from soil is frequently immense. Further, some soil types can exacerbate the problem by tightly binding with contaminants. Soils with a heavy clay content are frequently exceedingly difficult to decontaminate because of such binding actions.
Interestingly, in many contamination sites there will exist naturally-occurring microorganisms which have a capacity to aid in removal of the contaminant from the environment. Methods by which such microorganisms can aid in removal of contaminants vary. Sometimes, the contaminants are actually degraded, either partially or totally, by the microorganisms. Othertimes, the microorganisms can convert a contaminant to a substance which binds less tightly to the soil and is therefore easier to remove from the soil. In addition to the naturally-occurring microorganisms which may aid in removal of contaminants, non-naturally-occurring microorganisms may also be utilized. Such non-naturally-occurring microorganisms may sometimes be created by skilled scientists for the purpose of removing a contaminant from the environment. Example microorganisms which may be useful in remediating contaminated soils are described in U.S. Pat. No. 5,387,271, to Crawford et. al., entitled "Biological System For Degrading Nitroaromatics In Water And Soils," which is incorporated herein by reference.
In spite of the knowledge that naturally occurring and non-naturally-occurring microorganisms can aid in removal of contaminants from soils, and, in spite of frequent speculation that such microorganisms may be useful in remediating contaminated soil sites, it has been a considerable challenge to develop devices and procedures which can efficiently tap the utilities of such microorganisms. Among the problems faced are: 1) the microorganisms are frequently anaerobic so that oxygen must be substantially excluded from the environment of the microorganisms if they are to function efficiently; 2) the microorganisms, or some substance formed by the microorganisms, must generally contact a contaminant before the microorganisms can efficiently aid in removing the contaminant, so there must be efficient mixing of the microorganisms with a contaminated soil; and 3) the contaminated sites are generally enormous, possibly several square miles or larger in size. It is desirable therefore to develop methods and apparatuses which can be used in conjunction with microbiological activity to clean up contaminated soil sites.
Another set of problems facing those who would remediate contaminated soil sites concern the difficulties in preventing spillage of contaminated soil during the remediation process. Spilled contaminated soil may contaminate areas that were previously clean. Such spillage is particularly likely to occur during transport of the contaminated soil, as the soil may become a dust which is wind-blown to clean areas, or may be dribbled from open containers, or trucks, passing over the clean areas. Accordingly, it would be desirable to develop methods and apparatuses which minimize spread of contaminated soil from a remediation site during a remediation process.
Yet another set of problems facing those who would remediate contaminated soil sites arises from the remote locations of the sites. Frequently, such sites lack access to electrical power and lack nearby facilities for repair of broken equipment. Accordingly, it would be desirable to minimize the power requirements of decontamination apparatuses utilized at the sites, and to provide relatively durable decontamination apparatuses.
One method for remediating a site is to excavate the contaminated soil, haul it to a treatment vessel, decontaminate the soil in the treatment vessel, and then return the soil to the environment preferably using it to refill the site from which it was originally excavated. The reason for hauling the soil to a treatment vessel, rather than treating it in situ, is that a treatment vessel provides better control over the decontamination conditions. Such better control can lead to better decontamination of a soil.
A particular type of treatment vessel which may be used for decontaminating soils is a so-called "bioreactor vessel," which is a vessel within which microorganisms, or microorganism by-products, are utilized during the decontamination process. Example microorganisms which may be used in a bioreactor vessel are described in the U.S. Pat. No. 5,387,271. The microorganisms described in U.S. Pat. No. 5,387,271 require substantially anaerobic conditions during a degradation process. Such a requirement for anaerobic conditions during a microbial degradation process is not unusual, so it is frequently desirable to maintain substantially anaerobic conditions during microbial decontamination of a soil.
One method for maintaining such substantially anaerobic conditions, described generally in U.S. Pat. No. 5,387,271, is to mix aerobic microorganisms with the anaerobic microorganisms which will degrade the contaminant. The aerobic microorganisms scavenge oxygen from within the bioreactor vessel to render the vessel substantially anaerobic and thereby provide conditions suitable for the anaerobic microorganisms to degrade the contaminant. As long as sufficient nutrients are present to sustain the aerobic microorganisms, they will do an exceptional job of scavenging oxygen, possibly permitting a fluid within a bioreactor vessel to be left open to the atmosphere during the anaerobic degradation process. A fluid 12 comprising aerobic microorganisms and suitable nutrient is illustrated in FIG. 1.
Referring to FIG. 1, a bioreactor vessel 10 containing a fluid 12 is shown in cross-sectional side view. Fluid 12 comprises both aerobic and anaerobic microorganisms, a food source for the microorganisms, and a contaminant which is to be degraded by the anaerobic microorganisms and which will only be degraded under substantially anaerobic conditions. The bioreactor vessel comprises sides 14 and a bottom 16, but comprises no top. Accordingly, a top surface 18 of fluid 12 is exposed to the atmosphere 19, and thereby exposed to oxygen.
Due to the action of the aerobic microorganisms, oxygen which diffuses through top surface 18 of fluid 12 is quickly scavenged. Accordingly, fluid 12 becomes stratified into a substantially anaerobic lower portion 20 and a partially aerobic upper portion 22 wherein oxygen has diffused into fluid 12 and is being scavenged by the aerobic microorganisms.
A difficulty presented when soil slurries are to be decontaminated is that the soil of the slurry will settle to the bottom 16 of bioreactor vessel 10. Once the soil settles, the substantially anaerobic layer 20 is subdivided into a soil layer 24 at the bottom of the vessel, and a relatively clear layer 23 above the soil layer 24. The soil layer 24 generally takes an exceedingly long time to decontaminate because there is relatively poor mixing of microorganisms and microorganism nutrients within the contaminated soil. Also, the soil layer 24 can become quite compact, and therefore vigorous mixing may be required to uniformly disperse the soil into contact with the anaerobic microorganisms. A difficulty presented by such vigorous mixing is that the partially aerobic layer 22 may be disturbed by the mixing so that oxygen becomes dispersed throughout an entire depth of fluid 12 and thereby inhibits the decontamination activity of the anaerobic microorganisms. Accordingly, it would be desirable to develop a mixing system which could vigorously mix the lower layer 20 of fluid 12 while having substantially minimal dispersion of oxygen from level 22 into the lower level 20. It would be most desirable to develop a mixing system which enabled anaerobic microorganisms to continue to function during mixing of the lower layer 20 of fluid 12.
Another difficulty of decontaminating a soil slurry is that if the contaminated soil is not entirely exposed to substantially equal amounts of decontamination activity, certain pockets of the soil will remain contaminated while the rest of the soil becomes decontaminated. If these contaminated pockets are returned to the environment with the decontaminated soil, the remediation site may not be adequately decontaminated by a decontamination process. Accordingly, it would be desirable to develop a process which vigorously and uniformly disperses the soil layer 24 so that substantially all of the contaminated soil is exposed to substantially equal amounts of decontamination activity.
Other difficulties of decontaminating a soil slurry in a bioreactor vessel arise from the massive scale of the decontamination processes. Typical bioreactor vessels are on the order of about 30 feet wide by about 100 feet long by 3 to 5 feet deep. These vessels may be formed either above ground, as tanks, or below ground, as lined ponds. In either event, the vessels are commonly referred to as bioreactor "ponds" because of their large size. The large size of the bioreactor vessels creates substantial challenges in mixing the content of the vessels. A prior art method for mixing the content has been to provide walkways over the top of the vessels and to have persons manually move a "wand" through the content of the vessel to stir the content. The wand is generally a hollow tube connected to a pump and in fluid communication with an inlet tube extending into the fluid layer 20 of the pond. The inlet is preferably kept in the relatively clear fluid layer 23 above the soil layer 24 to lessen the probability of sucking particulate matter into the tube, as such particulate matter is likely to eventually plug the tube.
The fluid sucked into the inlet tube is expelled through the wand with a force generated by the pump. The wand generally comprises a nozzle, and this nozzle is ideally held in proximity to, or within, the soil layer 24. Thus, fluid expelled through the nozzle generates a mixing action within the layer 24 which disperses previously settled soil throughout the fluid layer 20.
Several problems are presented by this prior art method. Among these problems has been inadequate uniformity of the mixing of layer 24. A person manually moving a wand within the soil layer 24 generally does not uniformly mix the soil.
Another problem has been that persons will occasionally punch the nozzle of the wand through the soil layer 24 and into a lining at the bottom of the bioreactor vessel to thereby create a leak in the bottom of the vessel. As the material contained within the vessel is a contaminated material, it is generally undesired to have such contaminated material spilling from the bioreactor vessel into the adjacent environment.
Other prior art methods of mixing include mechanical agitation of a fluid by movement of a paddle or blade through the fluid. Such mechanical agitation is generally difficult to utilize in soil-slurry pond or vessel due to an inherent risk of punching through a lining of the pond or vessel.
For the above-discussed reasons, it would be desirable to develop an alternate mixing system for bioreactor vessels.